Red paint for ceramic decoration

ABSTRACT

The present invention provides a red paint for ceramic decoration, including a glass matrix, and a red colorant and a protective material that are intermingled in the glass matrix. The red colorant contains gold nanoparticles and silver nanoparticles. The protective material contains silica nanoparticles.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2017/010934 filed on Mar. 17, 2017, which claims priority toJapanese Application No. 2016-068160 filed on Mar. 30, 2016, thecontents of which are hereby incorporated by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a red paint for ceramic decoration.More particularly, the present invention relates to a red paintcontaining nanogold (Au) particles and nanosilver (Ag) particles as ared colorant.

The present application claims priority based on Japanese PatentApplication No. 2016-068160, filed on Mar. 30, 2016, the entire contentsof which are incorporated herein by reference.

BACKGROUND ART

A production example of ceramics typified by chinaware, tiles and thelike involves firstly kneading and molding prepared earths such askaolin, silica sand, and feldspar, as starting materials, followed bydrying and baking to produce a substrate. Then a glaze or paint isapplied to the surface of the substrate, and decoration firing isperformed. As a result, a decorated ceramic having a color or patternburnt onto the surface thereof can be obtained.

Red is a highly demanded color in ceramic decoration, by virtue of beingassociated with national flags, corporate logos, characters, Christmasand so forth. Cadmium-based materials such as selenium-cadmium red havebeen widely used conventionally as red colorants contained in redpaints, since such materials afford a vivid red coloration. There is atrend however towards restricting the use of selenium-cadmium red, dueto the toxicity of cadmium.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Publication No.    2013-023735

SUMMARY OF INVENTION

In view of the above considerations, it is an object of the presentinvention to provide a red paint for ceramic decoration (painting) thataffords for a vivid red coloration.

The present invention provides a red paint for ceramic decoration,including a glass matrix, and a red colorant and a protective materialthat are intermingled in the glass matrix. The red colorant containsgold nanoparticles and silver nanoparticles. The protective materialcontains silica nanoparticles.

The gold nanoparticles elicit a bluish red (red-purple) coloration,referred to as “maroon”, upon absorption of green to light-blue light.The silver nanoparticles afford a yellow coloration through absorptionof blue light. Accordingly, the concomitant use of gold nanoparticlesand silver nanoparticles as a red colorant allows the silvernanoparticles to absorb the blue light in the “maroon” derived from thegold nanoparticles. A vivid red coloration can be achieved as a result.Further, dispersing the red colorant in the glass matrix together withthe protective material makes the red colorant less likely to penetrateinto the glass matrix during decoration firing, and allows achievingstably a crisply tinged red coloration.

Unless otherwise stated, the term “nanoparticles” in the presentspecification denotes particles having an arithmetic mean value (D50particle size) of 1 to 100 nm of an equivalent circle diameter based onan electron microscopy, for instance a transmission electron microscope(TEM).

Unless otherwise stated, the term “ceramic” in the present specificationdenotes nonmetallic inorganic materials in general. For instancechinaware materials, glass materials and so forth are typical examplesencompassed within the term ceramics in the present specification.

Patent Literature 1 discloses a red glaze containing coppernanoparticles. In Patent Literature 1, a red coloration is achievedthrough decoration firing of the above red glaze in an oxidizingatmosphere. However, not a little copper oxide (II) is generated,through oxidation of copper, when the copper nanoparticles disclosed inPatent Literature 1 are fired in an oxidizing atmosphere (for instancean air atmosphere). When using the above red glaze, therefore, a reddecorative portion takes on dark, dull color tone derived frominsufficient brightness or insufficient yellow tinge (for instance, L*value and b* value in the L*a*b* color system). Therefore, the above thered glaze failed to become an alternative of for instanceselenium-cadmium red, which affords a vivid red.

In a preferred embodiment of the red paint disclosed herein, the volumeratio of the gold nanoparticles and the silver nanoparticles satisfiesgold nanoparticles:silver nanoparticles=80:20 to 20:80. As a result theeffect of using concomitantly gold nanoparticles and silvernanoparticles can be brought out to a higher level, and a vivid redcoloration can be achieved yet more stably.

In a preferred embodiment of the red paint disclosed herein, theproportion of the gold nanoparticles is 0.05 parts by volume to 0.5parts by volume with respect to 100 parts by volume as the total of theglass matrix and the protective material. The a* value in the reddirection in the L*a*b* increases as a result, and red color developmentcan be enhanced thereby. Further, the brightness L* value in the L*a*b*color system increases, and there can be suitably achieved a brightvivid tinge. Moreover, costs can be kept low by preventing the red paintfrom becoming excessively expensive.

In a preferred embodiment of the red paint disclosed herein, theproportion of the silver nanoparticles is 0.05 parts by volume to 0.4parts by volume with respect to 100 parts by volume as the total of theglass matrix and the protective material. As a result there increasesthe b* value in the yellow direction, in the L*a*b* color system, whichallows suppressing bluishness in the gold nanoparticles at a high level.A yet sharper red color development can therefore be achieved. Moreover,costs can be kept low by preventing the red paint from becomingexcessively expensive.

In a preferred embodiment of the red paint disclosed herein, the volumeof the protective material is 20 times or more the volume of the redcolorant. As a result, this renders penetration of the red colorant intothe glass matrix during decoration firing yet unlikelier, and red fadingis better suppressed. Excellent red color development can be realized asa result at a yet higher level.

In a preferred embodiment of the red paint disclosed herein, theproportion of the protective material is 10 vol % to 40 vol % withrespect to 100 vol % as the total of the glass matrix and the protectivematerial. In addition to red color development, a red decorative portionof high specular glossiness and of appearance excellent in gloss feelingcan be achieved as a result.

In another aspect the present invention provides also a ceramic producthaving a red decorative portion. The red decorative portion containsglass, gold and silver. The red decorative portion satisfies thefollowing conditions in the L*a*b* color system, based on JapaneseIndustrial Standards JIS Z8729 (2004): the L* value is 35 to 70; the a*value is 20 or greater; and the b* value is 15 or greater. In onepreferred embodiment, a 45-degree specular glossiness of the reddecorative portion based on Japanese Industrial Standards JIS Z8741(1997) is 70% or higher.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart for explaining a method for producing a red paintaccording to an embodiment of the present invention.

FIG. 2 is a matrix denoting the relationship between the content ratioof red colorant and brightness and chromaticity in the L*a*b* colorsystem.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be explained below.Any features other than the features specifically set forth in thepresent description and which may be necessary for carrying out thepresent invention can be regarded as instances of design matter, for aperson skilled in the art, based on conventional techniques in thetechnical field in question. The present invention can be realized onthe basis of the disclosure of the present specification and commontechnical knowledge in the relevant technical field. In the presentspecification a numerical value range notated as A to B (where A and Bare arbitrary numerical values) denotes a value equal to or larger thanA and equal to or smaller than B.

<<Red Paint>>

The red paint disclosed herein is a red paint for forming a reddecorative portion in a ceramic, as an object to be decorated, bycoating the surface of the ceramic with the red paint and by performingthen decoration firing. The red paint has a glass matrix, and a redcolorant and a protective material dispersed in the glass matrix,wherein the red colorant contains nanogold (Au) particles and nanosilver(Ag) particles, and the protective material contains silicananoparticles. Other properties are therefore not particularly limited,and can be arbitrary established in the light of various criteria, forinstance through addition of various components and modification of thecompositions thereof. The various constituent components will beexplained next in order.

The glass matrix has matrix properties of dispersing the red colorantand the protective material. The red colorant and the protectivematerial are intermingled in the glass matrix. The glass component, thered colorant component and the protective material component aretypically in the form of a sintered compact resulting from integralsintering of the foregoing.

The glass matrix is a component functioning as an inorganic binder ofthe red colorant. The glass matrix has also the function of enhancingbonding between the red colorant and the ceramic being the object to bedecorated. The coefficient of linear thermal expansion (averagecoefficient of linear expansion measured in a temperature region from25° C. to 500° C. using a thermomechanical analyzer; likewise hereafter)of the glass that makes up the glass matrix is not particularly limited,and may be identical to that of the ceramic as the object to bedecorated. As an example, the coefficient of linear thermal expansion ofthe glass may be about (coefficient of thermal expansion of the objectto be decorated) ±2×10⁻⁶K⁻¹. The coefficient of linear thermal expansionof the glass can lie for instance in the range of 4.0×10⁻⁶K⁻¹ to8.0×10⁻⁶K⁻¹. As a result the difference between the shrinkage factors ofthe object to be decorated (ceramic) and of the red paint at the time ofdecoration firing becomes smaller, and the occurrence of peeling,cracking and so forth at the site decorated with the red colorantbecomes unlikelier.

The glass transition point (Tg value based on differential scanningcalorimetry; likewise hereafter) of the glass that makes up the glassmatrix is not particularly limited. For instance, the glass transitionpoint may be of about 400° C. to 1500° C., on the basis of thebelow-described relationship with respect to sintering temperature.Within the above range, the glass transition point may be about 900° C.to 1300° C. in underglaze decoration applications and sink-inapplications. The glass transition point in overglaze decorationapplications may be about 500° C. to 900° C.

Examples of glass types that can have such properties (coefficient ofthermal expansion and glass transition point) include for instanceSiO₂—RO glass (where RO represents an oxide of a group II element, forinstance MgO, CaO, SrO and BaO; likewise hereafter), SiO₂—RO—R₂O glass(where R₂O represents an oxide of an alkali metal element, for instanceLi₂O, Na₂O, K₂O, Rb₂O, Cs₂O and Fr₂O, in particular Li₂O; likewisehereafter), SiO₂—RO—ZnO glass, SiO₂—RO—ZrO₂ glass, SiO₂—RO—Al₂O₃ glass,SiO₂—RO—Bi₂O₃ glass, SiO₂—R₂O glass, SiO₂—ZnO glass, SiO₂—ZrO₂ glass,SiO₂—Al₂O₃ glass, RO—R₂O glass, RO—ZnO glass or the like. The foregoingtypes of glass may contain one, or two or more components in addition tothe main constituent components given in the above denomination. Theglass may be ordinary amorphous glass, or crystal-containingcrystallized glass.

In a preferred embodiment, SiO₂ is half (50 mol %) or more with respectto 100 mol % as the whole glass. The glass transition point tendsgenerally to increase with rising proportion of SiO₂. Accordingly, theproportion of SiO₂ may be kept at or below about 80 mol % in the casefor instance where the below-described sintering temperature is set tobe low.

Adding components such as RO, R₂O, and B₂O₃ is effective from theviewpoint of lowering the glass transition point and thereby enhancingthe meltability of the glass. By contrast, the coefficient of thermalexpansion of the glass tends to become higher with increasing content ofthe foregoing components. In a preferred embodiment, RO is about 15 to35 mol % with respect to 100 mol % as the whole glass. In anotherpreferred embodiment, R₂O is 0 to 5 mol % with respect to 100 mol % asthe whole glass.

In a preferred embodiment, the glass is made up of a multi-componentsystem of four or more components (for instance, five or morecomponents). The physical stability of the red decorative portion isenhanced as a result. In applications that involve for instancedecoration of tableware, the red decorative portion is required toexhibit sufficient acid resistance towards acidic foods and sufficientalkali resistance towards alkaline detergents. In these cases, it iseffective to add for instance a component such as Al₂O₃, ZnO, and CaO,for instance in a proportion of 1 mol % or higher, in addition to themain constituent components given in the above denomination. Thechemical durability of the red decorative portion can be better enhancedas a result. Wear resistance as well can be preferably enhanced.

In a preferred embodiment, the glass contains substantially no componentthat may adversely impact human health or the environment, for instancearsenic components, lead components or cadmium components. Inapplications of tableware decoration, in particular, it is preferablenot to actively add any such components (which may however bepermissibly mixed into the glass as unavoidable impurities).

The proportion of the glass matrix in the red paint is not particularlylimited. The proportion of the glass matrix is about 50 vol % or higher,typically 60 vol % or higher and for instance 70 vol % or higher, withrespect to 100 vol % as the whole red paint, from the viewpoint ofenhancing bonding with the ceramic, being the object to be decorated.The proportion of the glass matrix is about 95 vol % or lower, typically90 vol % or lower, and for instance 80 vol % or lower, with respect to100 vol % as the whole red paint, from the viewpoint of enhancing redcolor development. For identical reasons the proportion of the glassmatrix is about 50 vol % or higher, typically 60 vol % or higher, andinstance 70 vol % or higher, and is about 95 vol % or lower, typically90 vol % or lower, and for instance 80 vol % or lower, with respect to100 vol % as the total of the glass matrix and the protective material.

The red colorant is a component that imparts red color to the surface ofthe ceramic being the object to be decorated. The red colorant isintermingled with the protective material in the glass matrix. The redcolorant and the protective material may be present independently fromeach other, or may be present as clumps resulting from aggregation ofthe protective material around the red colorant. The red colorantcontains gold nanoparticles and silver nanoparticles as metalnanoparticles.

The metal nanoparticles exhibit specific optical features (for instancestrong light absorption bands) in the ultraviolet to visible regions,arising from surface plasmon resonance (SPR). For instance nanogold (Au)particles absorb light of a wavelength in the vicinity of 530 nm (greenlight to light-blue light) and exhibit a bluefish red (red-purple)coloration referred to as “maroon”. For instance nanosilver (Ag)particles absorb light (blue light) at a wavelength in the vicinity of420 nm and exhibit yellow coloration. Through the concomitantly use ofgold nanoparticles and silver nanoparticles, accordingly, the bluishwavelength of gold nanoparticles is absorbed by the silvernanoparticles, and a vivid red coloration is brought about thereby. Thegold nanoparticles and the silver nanoparticles may be in the form ofrespective single particles, or may be in an alloyed state. So long asthe effect of the technology disclosed herein is not significantlyimpaired thereby, the red colorant may include metal nanoparticles otherthan gold and silver, for instance at a volume proportion no greaterthan 1/10 of the total volume of gold nanoparticles and silvernanoparticles.

The D50 particle size of the gold nanoparticles and the silvernanoparticles is a nanometer size (about 1 to 100 nm), in terms of atrade-off with the above surface plasmon resonance. The surface plasmonresonance effect of metal nanoparticles can vary depending on particlesize. The surface plasmon resonance effect can be better exploitedthrough proper adjustment of particle size to a nanometer size.

In a preferred embodiment, each of the D50 particle size of the goldnanoparticles and silver nanoparticles is 5 nm or greater, typically 10nm or greater and for instance 15 nm or greater. In another preferredembodiment, each of the D50 particle size of the gold nanoparticles andsilver nanoparticles is about 80 nm or smaller, typically 50 nm orsmaller, and for instance 30 nm or smaller. By prescribing the D50particle size to lie within the above ranges allows increasingabsorbance at the specific wavelengths of the metal nanoparticles, whileachieving good red coloration with small addition amounts. Further,precise decoration can be realized, with little color unevenness.

The mixing ratio of the gold nanoparticles and silver nanoparticles isaffected also for instance by the particle size and particle sizedistribution of the metal nanoparticles, and accordingly is notparticularly limited. For instance, the volume ratio may be set so thatthe volume of the gold nanoparticles is greater, or so that the volumeof silver nanoparticles is greater. In a preferred embodiment, thevolume ratio of gold nanoparticles and silver nanoparticles satisfiesgold nanoparticles:silver nanoparticles=80:20 to 20:80, for instancegold nanoparticles:silver nanoparticles=72:28 to 36:64. The effect ofthe technology disclosed herein can be brought out at a high level as aresult, and a vivid red coloration can be realized yet more stably.

The proportion of the red colorant in the red paint is not particularlylimited. The proportion of red colorant is about 0.1 vol % or higher,for instance 0.15 vol % or higher, and is about 1 vol % or lower,typically 0.8 vol % or lower, and for instance 0.7 vol % or lower, withrespect to 100 vol % as the whole red paint, for instance from theviewpoint of enhancing red color development and in terms of cost. Foridentical reasons, the proportion of red colorant may be about 0.1 partsby volume or more, for instance 0.15 parts by volume or more, and may beabout 1 part by volume or less, typically 0.8 parts by volume or lessand for instance 0.7 parts by volume or less, with respect to 100 partsby volume as the total of the glass matrix and the protective material.

In a preferred embodiment, the volume of the red colorant is about 1/100to 1/200, for instance 1/120 to 1/180, of the volume of the glassmatrix. A homogeneous red coloration can be achieved stably as a result.

In a preferred embodiment, the proportion of gold nanoparticles is about0.05 parts by volume or more, preferably 0.1 parts by volume or more,and for instance 0.11 parts by volume or more, with respect to 100 partsby volume as the total of the glass matrix and the protective material.By prescribing the proportion of gold nanoparticles to be equal to orgreater than a predetermined value it becomes possible to increase thea* value in the red direction, in the L*a*b* color system, and toenhance thus red color development. Further, the brightness L* value canalso be increased, and a bright vivid tinge can be better achieved.

In another preferred embodiment, the proportion of gold nanoparticles isabout 0.5 parts by volume or less, typically 0.45 parts by volume orless, preferably 0.35 parts by volume or less, and for instance 0.32parts by volume or less, with respect to 100 parts by volume as thetotal of the glass matrix and the protective material. By prescribingthe proportion of gold nanoparticles to be equal to or smaller than apredetermined value it becomes possible to increase the brightness L*value in the L*a*b* color system, and to better realize a bright vividtinge. Costs can moreover be kept low.

In a preferred embodiment, the proportion of silver nanoparticles isabout 0.05 parts by volume or more, preferably 0.1 parts by volume ormore, and for instance 0.11 parts by volume or more, with respect to 100parts by volume as the total of the glass matrix and the protectivematerial. By prescribing the proportion of silver nanoparticles to beequal to or greater than a predetermined value it becomes possible toincrease the b* value in the yellow direction, in the L*a*b* colorsystem, and to achieve yet superior red color development.

In another preferred embodiment, the proportion of silver nanoparticlesis about 0.4 parts by volume or less, preferably 0.35 parts by volume orless, and for instance 0.32 parts by volume or less, with respect to 100parts by volume as the total of the glass matrix and the protectivematerial. By prescribing the proportion of silver nanoparticles to beequal to or smaller than a predetermined value it becomes possible toincrease the brightness L* value in the L*a*b* color system, and tobetter realize a bright vivid tinge. Costs can moreover be kept low.

The protective material is a component for suppressing fading of the redcolorant during decoration firing and for enhancing red colordevelopment. Specifically, the protective material has a higher meltingpoint than a below-described decoration firing temperature. As a resultthe red colorant (nanometal) and glass do not come readily into contactwith each other during decoration firing, thanks to the fact that thered paint contains the protective material. In consequence it becomespossible to prevent the red colorant from being taken up as aconstituent component of the glass, in other words, to suppressdissolution of the red colorant in the glass. The state in which the redcolorant is dispersed in the glass in the form of metal particles, alsoafter decoration firing, can be maintained stably as a result. Thereforea crisply tinged red coloration can be achieved that boasts excellentcolor development. The protective material is intermingled with the redcolorant in the glass matrix. The protective material may be in a stateseparate from that of the red colorant, or may be for instance adheredto, bonded to or coordinated with the surface of the red colorant.

The protective material contains silica nanoparticles. The transparencyof silica increases when sintered. An effect is elicited as a result ofaccentuating color development in the red decorative portion, andreinforcing a gloss feeling. Further, silica is commercially availableat a comparatively low expense, and accordingly is preferable in termsof availability and cost. So long as the effect of the technologydisclosed herein is not significantly impaired thereby, the protectivematerial may contain ceramic nanoparticles other than silicananoparticles, for instance at a volume proportion lower than that ofthe silica nanoparticles. Concrete examples thereof include for instancezirconia nanoparticles, alumina nanoparticles and titania nanoparticles.

The D50 particle size of the protective material is of nanometer size(about 1 to 100 nm). The D50 particle size of the protective material(typically silica nanoparticles) is typically identical to, or smallerthan, the D50 particle size of the metal nanoparticles. In a preferredembodiment, the D50 particle size of the protective material is about 50nm or smaller, typically 30 nm or smaller and for instance 20 nm orsmaller. By prescribing the D50 particle size to be equal to or smallerthan a predetermined value, the specific surface area of the particlesize increases, and there is better elicited the effect of suppressingfading of the red colorant during decoration firing. It becomestherefore possible to achieve a superior red coloration yet more stably.

In a preferred embodiment, the volume of the protective material isabout 5 times or more, typically 10 times or more, preferably 20 timesor more, for instance 30 times or more the volume of the red colorant.The effect of suppressing fading of the red colorant during decorationfiring can be better brought out as a result.

In another preferred embodiment, the volume of the protective materialis about 90 times or less, typically 80 times or less, preferably 70times or less, for instance 60 times or less, and more preferably 50times or less the volume of the red colorant. As a result there can berealized a red decorative portion of better enhanced appearance (glossfeeling, glow, and color tone).

The proportion of the protective material in the red paint is notparticularly limited. The proportion of the protective material may beabout 10 vol % or higher, for instance 20 vol % or higher, with respectto 100 vol % as the whole red paint, from the viewpoint of enhancing redcolor development. The proportion of the protective material may beabout 40 vol % or lower, for instance 30 vol % or lower, with respect to100 vol % of the whole red paint, from the viewpoint of enhancing glossfeeling or glow. For similar reasons, the proportion of the protectivematerial may be about 10 vol % or higher, for instance 20 vol % orhigher, and may be about 40 vol % or lower, for instance 30 vol % orlower, with respect to 100 vol % as the total of the glass matrix andthe protective material.

The red paint disclosed herein may be made up of the above threecomponents (glass, red colorant and protective material), or may haveadded thereto other components, as appropriate, for instance within arange of less than about 10 vol %, with respect to 100 vol % as thewhole red paint, so long as the effect of the technology disclosedherein is not significantly impaired in doing so. Examples of additionalcomponents include for instance organic binders, reaction accelerators,surfactants, dispersants, thickeners, pH adjusters, preservatives,antifoaming agents, plasticizers, stabilizers, antioxidants and thelike. In a preferred embodiment, the red paint contains substantially nocomponent that may adversely impact human health or the environment, forinstance arsenic components, lead components or cadmium components(which may however be permissibly mixed into the red paint asunavoidable impurities). In applications of tableware decoration, inparticular, it is preferable that the red paint contains no suchcomponents.

The red paint disclosed herein can be adjusted in various ways dependingon the intended application. For instance, the red paint can be preparedin cullet form, powder form, frit form, pellet form, plate form or pasteform. As an example in a case for instance where fine decoration is tobe imparted to the surface of the object to be decorated, the red paintmay be prepared as a paste form through addition of a solvent (forinstance an aqueous solvent) to the red paint.

<<Method for Producing the Red Paint>>

The method for producing such a red paint is not particularly limited,and for instance may involve adding glass frit to a mixture of the redcolorant and the protective material, with further mixing, drying andintegral sintering, followed by pulverization, to thereby produce thered paint. The various steps will be explained next with reference tothe flowchart in FIG. 1.

The production method illustrated in FIG. 1 includes the followingsteps: (step S1) preparing a liquid first mixture through mixing of ared colorant and a protective material in accordance with a wet method;(step S2) mixing glass frit into the first mixture, to prepare a secondmixture; (step S3) subjecting the second mixture to a thermal treatment,to thereby yield a sintered compact in which the red colorant and theprotective material are intermingled in the glass matrix; and (step S4)pulverizing the sintered compact. This production method allowsproducing a red paint of the present embodiment in simple steps, i.e.mixing and a thermal treatment. Therefore, the above production methodis preferable in terms of convenience, workability and mass productivityas compared with for instance a production method that requires a stepof forming a silica coating film on the surface of metal nanoparticles.

In step S1 the red colorant and the protective material are mixed. Atleast gold nanoparticles and silver nanoparticles are prepared as thered colorant. At least silica nanoparticles are prepared as theprotective material. Nanoparticles have high cohesiveness, and hence aretypically commercially available in the form of dispersions in which theparticles have been stabilized in a dispersion solvent. In the presentembodiment, therefore, respective nanoparticle dispersions are weighedand mixed in such a manner that gold nanoparticles, silver nanoparticlesand silica nanoparticles obey predetermined volume ratios. The affinityof the red colorant and the protective material can be increased bymixing the red colorant and the protective material beforehand, prior toaddition of the glass frit. The mixing operation can be accomplished forinstance using a magnetic stirrer or relying on ultrasonic waves. Aliquid first mixture is thus prepared.

In step S2 glass frit is mixed, at a predetermined ratio, into theliquid first mixture. In the present embodiment a highly homogeneousmixture can be obtained through mixing of the first mixture and theglass frit in accordance with a wet method. A liquid second mixture isthus prepared.

In step S3 the second mixture is subjected to a thermal treatment. Forinstance, the second mixture is firstly dried at a temperature regionnot higher than 100° C., to remove the dispersion medium to a certainextent. The resulting mixture is next heated at a temperature equal toor higher than the glass transition point of the glass frit, to therebyintegrally sinter the mixture. The sintering temperature may be set tolie in the range of about a glass transition point +0° C. to 300° C. Inthe production for instance of a paint for overglaze decoration thesintering temperature may be set to about 800° C. to 900° C., in a casewhere the glass transition point of the glass frit is 600° C. to 800° C.The sintering time may be ordinarily set to range from about 0.1 toseveral hours. The atmosphere resorted to at the time of sintering maybe an air atmosphere, an oxidizing atmosphere, an inert gas atmosphereor the like. As a result of such a thermal treatment there is obtainedan integral sintered compact in which the red colorant and theprotective material are intermingled in the glass matrix.

In step S4 the sintered compact is pulverized (for instance crushed)and/or sorted to a desired size. The pulverization operation can beaccomplished for instance using a vibration mill, a planetary mill, astirring grinder or the like. The shape and size of the red paint arenot particularly limited, and for instance the average particle sizebased on laser diffraction-light scattering may be set to about 10 μm orless, typically in the range of 0.1 to 10 μm, and for instance about 0.5to 5 μm, for instance in terms of handleability.

The red paint thus obtained is used for decorating the surface of aceramic as the object to be decorated. The decoration operation may beaccomplished by applying the red paint onto the surface of a ceramic,followed by decoration firing at a predetermined temperature. Forinstance in “underglaze decoration”, where decoration is imparted to abaked substrate, and in “sink-in” where the red paint is incorporatedinto a glaze, the red paint may be applied to the surface of a ceramic,followed by decoration firing at high temperature, of about 1200° C. to1400° C. In “overglaze decoration”, where decoration is imparted to aglazed substrate (after glazing), the red paint may be applied to thesurface of the ceramic, followed by decoration firing at a mediumtemperature, of about 700° C. to 1000° C. The red paint of the presentembodiment can yield particularly good red coloration through firing atthe above medium temperature.

Decoration firing after application of the red paint can be carried outtypically in an air atmosphere (oxidizing atmosphere). For instancecinnabar (cupric oxide) widely used as a red colorant requiresdecoration firing under a reducing atmosphere. By contrast, the redpaint disclosed herein develops red coloration through firing in an airatmosphere, and accordingly no equipment is required for maintaining areducing atmosphere, which is advantageous in terms of ease of use.

A ceramic product having a red decorative portion can be thus obtainedas described above. The term “ceramic product” encompasses hereinpottery, porcelain, earthenware, stoneware, glass and so forth. Specificarticles include for instance tableware, decorative vessels, variouswall and floor tiles, sanitary wares, roof tiles, bricks, clay pipes,ceramic pipes and the like.

The red decorative portion of the ceramic product is a fired body thatcontains at least glass, gold and silver. The red decorative portion ofthe present embodiment affords as a result a unique vivid redcoloration. The appearance (color tone and gloss feeling) of this reddecorative portion allows further enhancing the aesthetics and premiumfeel of the ceramic, and contributes to providing articles that accruehigh customer satisfaction.

The color tone of the red decorative portion of the present embodimentin the L*a*b* color system according to JIS Z8729 (2004) can suitablysatisfy the following conditions:

the L* value lies in the range of 35 to 70 (preferably 35 to 55);

the a* value is 20 or greater (preferably 30 or greater, and forinstance 50 or smaller);

the b* value is 15 or greater (preferably 20 or greater, and forinstance 40 or smaller).

A bright vivid tinge can be realized by prescribing the brightness L*value to be equal to or greater than a predetermined value. A deep warmtinge can be realized by prescribing the brightness L* value to be equalto or smaller than a predetermined value. Red color development can beincreased, and a sharp crisp tinge can be realized, by prescribing thea* value in the red direction to be equal to or greater than apredetermined value. By prescribing the b* value in the yellow directionto be equal to or greater than a predetermined value, in other words bykeeping small the −b* value blue direction, it becomes possible torealize vivid red coloration by suppressing purplish to bluishcoloration, for instance as in “maroon” coloration.

The gloss feeling of the red decorative portion of the presentembodiment involves 45-degree specular glossiness according to JIS Z8741(1997), that can satisfy a value of 70% or higher, preferably 80% orhigher, and particularly preferably 90% or higher. As a result the reddecorative portion gives off glow when irradiated with light, whichtranslates into in an enhanced gloss feeling. Further, the smoothness ofthe surface increases and a luxurious beautiful appearance can beachieved.

Examples pertaining to the technology disclosed herein will be explainednext, but the invention is not meant to be limited to the examplesbelow.

<I. Assessment of Color Tone>

In the present test example gold nanoparticles and silver nanoparticleswere prepared as the red colorant, and color tone was assessed throughmodification of the content ratios of the gold nanoparticles and silvernanoparticles.

Specifically, firstly a dispersion (commercial product) of goldnanoparticles having a D50 particle size of 20 nm, a dispersion(commercial product) of silver nanoparticles having a D50 particle sizeof 20 nm and a dispersion (commercial product) of silica nanoparticleshaving a D50 particle size of 20 nm were mixed, to yield a first mixedsolution. Next, glass frit (glass transition point: 640° C.) having thecomposition given in Table 1 was added to the first mixed solution, withfurther mixing of the whole, to thereby yield a second mixed solution.The second mixed solution was dried in an oven, and thereafter wassubjected to a thermal treatment in an air atmosphere at a temperatureof 800° C. to 900° C. for 30 minutes, to thereby yield a sinteredcompact. Each obtained sintered compact was disintegrated using anIshikawa mixing and grinding machine, followed by pulverization using avibration mill and a planetary mill, in this order, to produce a powderyred paint having an average particle size of 0.5 to 5 μm.

The first mixed solution and the second mixed solution were prepared sothat, in the red paint state, glass was 80 vol %, silica as theprotective material was 20 vol %, and the gold nanoparticles (Au) andsilver nanoparticles (Ag), as the red colorant, were present in theproportion (parts by volume) given in Table 2 with respect to the total(100 parts by volume) of glass plus silica.

TABLE 1 Composition of glass frit SiO₂ CaO SrO BaO Li₂O Al₂O₃ ZnO ZrO₂SnO₂ (mol 57 11 8 6 3 4 9 1 1 %)

TABLE 2 Content ratio of red colorant and evaluation results Contentratio of red colorant Au Ag L*a*b* color system: evaluation results(pts. (pts. L* Determi- a* b* Determi- vol.) vol.) value nation 1 valuevalue nation 2 Ref. ex. 1 — 0.11 85.0 P 1.1 25.0 P Ref. ex. 2 — 0.2273.5 P 1.9 50.1 P Ref. ex. 3 0.28 — 40.5 G 46.3 4.8 P Ex. 1 0.60 0.4530.2 P 43.3 34.9 E Ex. 2 0.05 0.09 66.2 G 20.5 18.3 G Ex. 3 0.11 0.0558.1 G 32.9 15.8 G Ex. 4 0.11 0.11 52.2 E 33.8 24.3 E Ex. 5 0.22 0.1642.9 E 41.4 28.3 E Ex. 6 0.28 0.11 43.9 E 46.3 25.8 E Ex. 7 0.28 0.2239.6 E 46.9 35.3 E Ex. 8 0.32 0.32 37.6 E 46.4 37.3 E

Each produced red paint was applied to the surface of a ceramic (testpiece), and decoration firing was performed at 700° C. to 900° C. in anair atmosphere (oxidizing atmosphere), to thereby yield a respectiveceramic having a red decorative portion. Each red decorative portion wasmeasured for brightness (L*) and chromaticity (a*, b*) in the L*a*b*color system, according to JIS Z8729 (2004), using a spectrophotometerby Konica Minolta, Inc. The results are given in Table 2 in thecorresponding columns.

The columns “Determination” in Table 2 shows determination results basedon criteria (1) of Table 3. The column “Determination 1” in Table 2 setsout determination results of L* value. The column “Determination 1” setsout the notation “E (Excellent)” where the L* value is “0” (excellent),the notation “G (good)” where the L* value is “0” (good), and thenotation “P (Poor)” where the L* value is “X” (poor). The determinationresults of chromaticity (a*, b*) are given in the column “Determination2”. The column “Determination 2” shows the notation “E (Excellent)”where both the a* value and the b* value are “0” (excellent), thenotation “G (Good)” where one from among the a* value and b* value is“0” (excellent) and the other is “0” (Good), or where both the a* valueand b* value are “0” (good), and shows the notation “P (Poor)” where atleast one from among the a* value and b* value is “X” (poor).

TABLE 3 Criteria (1) X ◯ ⊗ Color L* value Smaller than 35 or 35 to 70 35to 55 tone greater than 70 a* value Smaller than 20 20 or greater 30 orgreater b* value Smaller than 15 15 or greater 20 or greater

FIG. 2 is a matrix denoting the relationship between content ratio ofred colorant and brightness and chromaticity in the L*a*b* color system.As Table 2 and FIG. 2 illustrate, a red decorative portion of vividcoloration having good chromaticity (a*, b*) could be realized using thered colorants in Examples 1 to 8, containing gold nanoparticles andsilver nanoparticles. Among the foregoing, brightness (L*) was enhancedand a bright vivid tinge could be better achieved through the use of thered paints in Examples 2 to 8, in which the proportion of goldnanoparticles and silver nanoparticles was 0.32 parts by volume or lesswith respect to the total of glass matrix plus protective material. Inparticular, bluishness was further suppressed and a red decorativeportion boasting a notably crisp red coloration could be achievedthrough the use of the red paints of Examples 4 to 8, in which theproportion of gold nanoparticles and silver nanoparticles lay in therange of 0.11 to 0.32 parts by volume with respect to the total of glassmatrix plus protective material.

<II. Assessment of Gloss Feeling>

In the present test example gold nanoparticles and silver nanoparticleswere prepared as the red colorant, and color tone was assessed throughmodification of the content ratios of the gold nanoparticles and silvernanoparticles. Red paints were obtained in the same way as in the aboveI, but herein the red paints were prepared in such a manner that, in thestate of the first mixed solution and the second mixed solution, theglass and the protective material were present in the proportions givenin Table 4, and the content of gold nanoparticles as the red colorantwas 0.3 parts by volume and the content of silver nanoparticles was 0.2parts by volume, with respect to the total (100 parts by volume) of theglass plus silica. A respective red decorative portion was formed usingeach red paint, and “appearance (color tone and gloss feeling)” wasmeasured.

The method for measuring color tone is the same as in the above I. Glossfeeling was measured in accordance with JIS Z8741 (1997) using a glossmeter manufactured by Nippon Denshoku Industries Co., Ltd. Table 5 setsout criteria for determining “Color tone” and “Glossiness”, thedetermination results being given in the corresponding columns of Table4. The criterion for color tone given in Table 5 is identical to thecriterion “0” (Excellent) in Test example I. Accordingly, instanceswhere the criterion in the column of color tone in Table 4 is satisfiedare shown as “E (Excellent)”. Instances that fail to satisfy thecriterion of Table 5 are noted with an asterisk (*) in the column ofglossiness of Table 4. In the comprehensive determination of Table 4,instances where color tone was “E” and glossiness was 90% or higher wererated as “E (Excellent)”, while instances where color tone was “E” andglossiness was 80% or higher were rated as “G (Good)”.

TABLE 4 Proportion of glass, protective material and red colorant Ex.Ex. Ex. Ex. Ex. Ex. 9 10 11 12 13 14 Glass (vol %) 95 90 80 70 60  50Protective material (vol %)  5 10 20 30 40  50 Protective material/redcolorant 10 20 40 60 80 100 Appearance Color tone — E E E E — Glossiness(%) 91 93 92 87 83 *32 Comprehensive — E E G G — determination

TABLE 5 Criteria (2) Criteria Color L* value 35 to 55 tone a* value 30or greater b* value 20 or greater Glossiness 70% or higher

As Table 4 reveals, red decorative portions where color tone andglossiness were both particularly good could be achieved in Examples 10to 13, in which the proportion of the protective material was 10 to 40vol % with respect to 100 vol % as the total of the glass matrix and theprotective material. Among the foregoing, a red decorative portion ofparticularly high gloss feeling, with a glossiness of 90% or higher,could be achieved using the red paints of Examples 10 and 11.

Concrete examples of the technology disclosed herein have been explainedin detail above, but the concrete examples are merely illustrative innature and are not meant to limit the scope of the claims in any way.The technology set forth in the claims includes variations andmodifications of the concrete examples illustrated above.

The invention claimed is:
 1. A red paint for ceramic decoration,comprising: a glass matrix; and a red colorant and a protective materialthat are intermingled in the glass matrix, wherein the red colorantcontains gold nanoparticles and silver nanoparticles, and the protectivematerial contains silica nanoparticles, and the volume of the protectivematerial is 20 times or more the volume of the red colorant.
 2. The redpaint according to claim 1, wherein the proportion of the protectivematerial is 10 vol % to 40 vol % with respect to 100 vol % as the totalof the glass matrix and the protective material.
 3. The red paintaccording to claim 2, wherein the volume ratio of the gold nanoparticlesand the silver nanoparticles satisfies gold nanoparticles:silvernanoparticles=80:20 to 20:80.
 4. The red paint according to claim 2,wherein the proportion of the gold nanoparticles is 0.05 parts by volumeto 0.5 parts by volume with respect to 100 parts by volume as the totalof the glass matrix and the protective material.
 5. The red paintaccording to claim 2, wherein the proportion of the silver nanoparticlesis 0.05 parts by volume to 0.4 parts by volume with respect to 100 partsby volume as the total of the glass matrix and the protective material.6. The red paint according to claim 2, wherein an arithmetic mean of thesizes of the gold nanoparticles, silver nanoparticles, and silicananoparticles sizes defines a value, wherein said value is a diameter ofan equivalent circle, and wherein said diameter is between one and onehundred nanometers.
 7. The red paint according to claim 6, wherein thesizes of the gold nanoparticles, silver nanoparticles, and silicananoparticles are D50 sizes measured with electron microscopy.
 8. Thered paint according to claim 1, wherein the volume ratio of the goldnanoparticles and the silver nanoparticles satisfies goldnanoparticles:silver nanoparticles=80:20 to 20:80.
 9. The red paintaccording to claim 1, wherein the proportion of the gold nanoparticlesis 0.05 parts by volume to 0.5 parts by volume with respect to 100 partsby volume as the total of the glass matrix and the protective material.10. The red paint according to claim 1, wherein the proportion of thesilver nanoparticles is 0.05 parts by volume to 0.4 parts by volume withrespect to 100 parts by volume as the total of the glass matrix and theprotective material.
 11. The red paint according to claim 1, wherein anarithmetic mean of the sizes of the gold nanoparticles, silvernanoparticles, and silica nanoparticles sizes defines a value, whereinsaid value is a diameter of an equivalent circle, and wherein saiddiameter is between one and one hundred nanometers.
 12. The red paintaccording to claim 11, wherein the sizes of the gold nanoparticles,silver nanoparticles, and silica nanoparticles are D50 sizes measuredwith electron microscopy.
 13. A red paint for ceramic decoration,comprising: a glass matrix; and a red colorant and a protective materialthat are intermingled in the glass matrix, wherein the red colorantcontains gold nanoparticles and silver nanoparticles, and the protectivematerial contains silica nanoparticles, and the proportion of theprotective material is 10 vol % to 40 vol % with respect to 100 vol % asthe total of the glass matrix and the protective material.
 14. The redpaint according to claim 13, wherein the volume ratio of the goldnanoparticles and the silver nanoparticles satisfies goldnanoparticles:silver nanoparticles=80:20 to 20:80.
 15. The red paintaccording to claim 13, wherein the proportion of the gold nanoparticlesis 0.05 parts by volume to 0.5 parts by volume with respect to 100 partsby volume as the total of the glass matrix and the protective material.16. The red paint according to claim 13, wherein the proportion of thesilver nanoparticles is 0.05 parts by volume to 0.4 parts by volume withrespect to 100 parts by volume as the total of the glass matrix and theprotective material.
 17. The red paint according to claim 13, wherein anarithmetic mean of the sizes of the gold nanoparticles, silvernanoparticles, and silica nanoparticles sizes defines a value, whereinsaid value is a diameter of an equivalent circle, and wherein saiddiameter is between one and one hundred nanometers.
 18. The red paintaccording to claim 17, wherein the sizes of the gold nanoparticles,silver nanoparticles, and silica nanoparticles are D50 sizes measuredwith electron microscopy.